1. Field of the Invention
The present invention relates to an electronic apparatus having a radio transmitter of a phase modulation type.
2. Description of the Related Art
FIG. 8 is a block diagram showing a configuration of a conventional computer system having a radio transmitter built therein. In an electronic apparatus having a radio transmitter, such as a computer system 101 having the radio transmitter shown in FIG. 8 built therein, power noise, which is not of such a level as to pose any problem for a digital circuit section 102 including a low-frequency logic circuit of the computer system in the case where no radio unit is used, has a conspicuous effect on the characteristics of a radio unit circuit 103, constituting a high-frequency circuit section of the radio transmitter, in the case where the radio unit is used. Especially in a radio transmitter of a phase modulation type, power noise has an adverse effect on the modulation accuracy, and therefore a series regulator generating only a small noise is preferably used as the power supply of the radio transmitter.
In the case where the series regulator is used as a power supply, however, the power efficiency of the power supply is deteriorated to cause an increased power consumption. For this reason, a switching regulator 104 is generally used as a power supply. The switching (SW) regulator 104 includes a PWM control circuit 106 for controlling the pulse width of the output pulse generated from a high-frequency transmission circuit (not shown) based on a reference voltage in response to the DC power from an AC/DC power supply or a DC power supply 105 such as a battery, and a switching & filter circuit 107 having a LC filter for averaging the pulse train obtained by switching the DC voltage through a switching element. The voltage output from the switching & filter circuit 107 is fed back to the PWM control circuit 106 which, in turn, controls the pulse width in such a manner as to eliminate the difference between the feedback voltage and the reference voltage, with the result that the switching & filter circuit 107 outputs a stable DC voltage.
In a computer system having built therein a radio transmitter employing a switching regulator as a power supply, however, the switching regulator 104 generates a ripple (switching) noise of a high power and frequency. Once the ripple frequency of the noise generated from this power supply approaches the comparison frequency sampled by the PLL built in the radio circuit of a phase modulation type, the modulation accuracy of the radio transmitter is adversely affected. The problem, therefore, is that in order to reduce the ripple noise of the power supply and thereby to prevent the comparison frequency of the PLL from being adversely affected by the ripple noise, an expensive ripple filter 108 occupying a large physical space must be added in the power line.
Japanese Unexamined Patent Publication Nos. 11-122123 and 6-303772 disclose techniques for avoiding the effect of the ripple noise on the radio transmitter. The technique disclosed by the former patent publication is for changing over the switching frequency when the receiving frequency of the radio receiver coincides with the switching frequency of the switching regulator, and the technique disclosed by the latter patent publication concerns the technique for changing the switching frequency upon detection of the generation of a wave interfering with the radio receiver such as a TV tuner. These techniques relate to the radio receiver but not to the modulation accuracy of the radio transmitter of phase modulation type. Also, in both techniques, the switching frequency of the switching regulator is changed over from the initially set frequency to another frequency upon generation of a ripple noise.